it's impossible to get actual numbers on this so it's difficult to speculate. but my impression is that most D/C fans skew younger, are therefore more active on tumblr and twitter than S/D shippers, and their voices are "louder" because they're the ones being active about it: tweeting, making youtube comments, posting on tumblr, etc.

however when you look at fanwork output on LJ, S/D and J2 work still dominate. and there's still a good number of wincest shippers on the websites I've mentioned previously. even more if you count in people who multiship.

so which one is actually more popular? I really don't know and I think it's an interesting question. don't let the wain in popularity get you down, keep shipping what you want to ship and loving S/D if that's what makes you happy.